We report the synthesis, structures, magnetic and luminescence properties of a series of three-blade propeller homoleptic lanthanide complexes [LnL₃](ClO₄)₃·H₂O·2MeCN (Ln = Eu (1), Tb (2), Dy (3), Er (4), Yb (5), Gd (6); L = 2,6-bis(1-methyl-imidazol-2-yl)pyridine). The Eu, Tb, and Dy analogues exhibit the characteristic lanthanide-based luminescence. Remarkably, the mixed Eu/Tb complex with a metal ratio of 1 : 10 can be used as a ratiometric thermometer in the solid-state between 130 and 220 K with the maximum value of S_r = 6.6% K⁻¹ within the range of 150–170 K. Theoretical calculations allow not only modelling of the luminescence properties of pure complexes but also rationalization of the performances of the mixed Eu/Tb thermometers.

Three novel lanthanide complexes with the ligand 4,4-difluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL), namely [LnL₃(H₂O)₂], Ln = Eu, Gd and Tb, were synthesized, and, according to single-crystal X-ray diffraction, are isostructural. The photoluminescent properties of these compounds, as well as of three series of mixed metal complexes [Eu_xTb_1-xL₃(H₂O)₂] (Eu_xTb_1-xL₃), [EuxGd_1-xL₃(H₂O)₂] (Eu_xGd_1-xL₃), and [Gd_xTb_1-xL₃(H₂O)₂] (Gd_xTb_1-xL₃), were studied. The Eu_xTb_1-xL₃ complexes exhibit the simultaneous emission of both Eu³⁺ and Tb³⁺ ions, and the luminescence color rapidly changes from green to red upon introducing even a small fraction of Eu³⁺. A detailed analysis of the luminescence decay made it possible to determine the observed radiative lifetimes of Tb³⁺ and Eu³⁺ and estimate the rate of excitation energy transfer between these ions. For this task, a simple approximation function was proposed. The values of the energy transfer rates determined independently from the luminescence decays of terbium(III) and europium(III) ions show a good correlation.

The effect of fluorinated chain length of Htta analogs in [Eu(L)₃phen] complexes on energy transfer was investigated and the complicated influence of substituents on the photophysical properties of the compounds was thoroughly discussed.